The present invention relates generally to a digital signal processing system and method especially adapted for reproducing and wave-shaping digital signals. More specifically, the invention relates to generation of a read-enable pulse for reproducing digital signals recorded on a magnetic medium, such as a magnetic tape, a magnetic disk and so forth.
As is well known, in digital signal recording, data encoded into the digital signal is recorded on a magnetic recording medium in the form of binary data by way of residual magnetic induction. In this case, it is not necessary to superimposed an AC bias on the recording signal.
It is also a well known technique to generate a read-enable pulse which orders reproduction of recorded digital information, differentiate the reproduced signal to derive a differentiated signal, and then detect the zero-crossing of the differentiated signal. In this case, the zero-crossing of the differentiated signal has to represent a peak in the reproduced signal. Therefore, by detecting every occurence of zero-crossing in the differentiated signal, the read-enable pulses can be generated synchronously with the peaks of the reproduced signal.
This conventional process for generating read-enable pulses works as long as the pulse intervals of the digital signal remain essentially constant and short enough that the analog reproduced signal is always changing due to magnetic reversals on the medium. However, in practice, the intervals between reversals vary significantly. When the pulse interval is significantly prolonged, the reproduced signal level tends to remain constant at approximately zero for a noticeable period of time before continuing toward the next amplitude peak, this waveform being commonly known as a "shoulder". When a shoulder appears in the reproduced signal, a saddle-point extremum with a zero amplitude appears in the differentiated signal saddle. This peak at the zero-amplitude level may be mistaken for a zero-crossing; this would lead to generation of a read-enable pulse at a timing that does not coincide with any of the peaks in the reproduced signal. This commonly causes errors in reading recorded data.
The probability of read error is especially high when the reproduced signal is noisy, in particular near shoulders.
This tendency can be ameliorated by applying MFM techniques to base data in order to hold the pulse intervals of the digital signal essentially constant. However, even in this case, variation among pulse intervals occurs due to reproduction velocity fluctuations when the recording medium is a magnetic disk. Therefore, this would not be sufficient to prevent read errors.